**Fresh water bony fishes maintain water balance by:**

A. **Excreting a hypotonic urine:**
Freshwater bony fishes face the challenge of constantly gaining water through their gills and body surface due to osmosis. To overcome this, they excrete large amounts of dilute, hypotonic urine. This helps them to eliminate excess water from their bodies and maintain a balance between their internal fluids and the surrounding freshwater environment.

B. **Drinking small amount of water:**
Freshwater fishes also regulate their water balance by drinking small amounts of water. By actively taking in water, they compensate for the constant loss of water through osmosis and maintain the necessary volume of water within their bodies.

C. **Excreting salt across their gills:**
Freshwater bony fishes are constantly exposed to a hypotonic environment, which leads to the influx of water into their bodies. To prevent excessive water uptake, they have specialized cells in their gills that actively transport salts (ions) out of their bodies. This helps them maintain the proper balance of ions and prevent osmotic imbalance.

D. **Excreting wastes in the form of uric acid:**
Freshwater fishes excrete waste products, such as ammonia and urea, primarily through their gills and kidneys. However, to conserve water, they convert toxic ammonia into less toxic uric acid, which is excreted as a solid waste. This process helps them minimize water loss and maintain a more efficient water balance.

In summary, freshwater bony fishes maintain water balance by excreting a hypotonic urine, drinking small amounts of water, excreting salt across their gills, and excreting wastes in the form of uric acid. These adaptations allow them to survive in a freshwater environment with constantly changing water concentrations and ensure their internal fluids remain in balance.